This invention relates to a method of heat treating polybenzazole fibers in order to improve the physical properties of the fibers.
Polybenzazole fibers, such as polybenzoxazole fibers, are expected to be the super fibers of the next generation because they can have a modulus two or more times higher than the modulus of poly-p-phenylene-terephthalamide fiber which is representative of super fibers on the market now.
The best modulus for polybenzazole fiber is not obtained unless the fiber is heat-treated. Conventional heat treatment methods are described in J. Mater. Sci., 20, 2727(1985) and H. D. Ledbetter, S. Rosenberg, C. W. Hurtig, Symposium Proceedings of The Materials Science and Engineering of Rigid-Rod Polymers, Vol. 134, 253 (1989). These conventional heat treatment processes for polybenzazole fibers must be conducted at temperatures of 500.degree. C. or more since the rigidity of the polybenzazole molecule is very high. Conventional fiber heat-treating equipment tends to be expensive, and the amount of time required can lead to heat treating becoming the bottle neck of industrial fiber production.
Heat treating is required in order to improve the modulus of polybenzazole fibers. This invention provides a new method of heat treatment which eliminates the necessity for prolonged high temperature heat treatment of polybenzazole fibers. Polybenzazole ("PBZ") fibers include fibers made of polybenzoxazole ("PBO") or polybenzothiazole ("PBT").